The present invention relates to using a high dielectric constant material to shape an electric field, and more particularly, a heating apparatus and method for heating plastic that incorporates such a material.
It is known to use RF (radio frequency) fields to soften plastic sheet materials, which materials are then formed by a mold into a desired shape. FIG. 1 shows a typical prior art apparatus for doing this. In particular, an RF generator 10, which typically provides a 10 to 100 MHz signal, is coupled to a pair of circular electrodes 12a and 12b. In turn, an RF field is created between circular (or other shape) cross-section electrodes 12 as indicated by field lines 14, which field passes through a sheet of plastic workpiece material 16. Sheet 16 typically has a thickness between about 1/8 to 1/4 inch (0.3175 to 0.635 cm). Such an apparatus heats most of sheet 16, although as can be seen by the spacing of field lines 14, not in a uniform manner. Thereafter sheet 14 is placed in male and female molds and pressed into the desired shape.
Frequently it is necessary to deform only a portion of the material in order to achieve the desired shape. Thus heating of the entire sheet is wasteful of the heating energy. Further, when the entire sheet is heated, the entire sheet must be supported since it is soft. Also incomplete female mold filling can result due to cooling of plastic, thereby making the molding of complex shapes difficult. A technique for localized heating that avoids some of these problems is to blow hot air just at the portions to be heated. However, the heat must then conduct through the plastic thickness before molding, which increases cycle time.
In contradistinction, in order to make some products, it is necessary to use a highly uniform RF field to achieve uniform softening. The typical prior art RF heating device of FIG. 1 undesirably concentrates the RF field at the portion of sheet 16 that is directly between electrodes 12.
It is therefore an object of the present invention to have an apparatus and method for RF heating of a plastic sheet wherein the RF field can be uniformly applied or locally applied, as desired, can make molding of complex shapes practical, and has a relatively short cycle time.